Polishing composition and method for polishing magnetic disk substrate

ABSTRACT

Embodiments of the invention provide a polishing composition including colloidal silica, pulverized wet-process silica particles, and a water-soluble polymer compound, wherein the water-soluble polymer compound is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid. Various embodiments achieve a high polishing rate and obtain a good surface smoothness and end-face shape without the use of alumina particles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority under 35 U.S.C. §119 to PCT Patent Application No. PCT/JP2015/058819, entitled. “POLISHING AGENT COMPOSITION AND METHOD FOR POLISHING MAGNETIC DISK SUBSTRATE,” filed on Mar. 24, 2015, which claims priority to Japanese Patent Application No. JP 2014-069366, entitled, “POLISHING AGENT COMPOSITION AND METHOD FOR POLISHING MAGNETIC DISK SUBSTRATE,” filed on Mar. 28, 2014, which are hereby incorporated by reference in their entirety into this application.

BACKGROUND Field

Embodiments of the invention relate to a polishing composition used for polishing electronic components such as semiconductors and magnetic recording media including hard disks. Particularly, embodiments of the invention relate to a polishing composition used for surface polishing of a substrate for a magnetic recording medium, such as a glass magnetic disk substrate and an aluminum magnetic disk substrate. Further, embodiments of the invention relate to a polishing composition used for surface polishing of an aluminum magnetic disk substrate for a magnetic recording medium, the substrate having a coating formed by electroless nickel-phosphorus plating on the surface of a substrate of an aluminum alloy.

In the past, use has been made of polishing compositions in which alumina particles with relatively large particle sizes which can provide a high polishing rate are dispersed in water from the viewpoint of productivity, as a polishing composition for polishing a surface coated by electroless nickel-phosphorus plating, of an aluminum magnetic disk substrate. However, the use of alumina particles has caused a problem that since alumina particles are considerably harder than a coating deposited by electroless nickel-phosphorus plating on an aluminum magnetic disk substrate, alumina particles are embedded in the substrate and the embedded particles have an adverse effect on the subsequent polishing step.

As solutions to this problem, polishing compositions have been proposed in which alumina particles and silica particles are combined. There have also been proposed methods in which polishing is performed, with no use of alumina particles, using only silica particles. Further, studies have been made on additives for reducing edge rounding (roll-off) a substrate when it is polished using only silica particles.

According to the conventional art, it is possible that by combining alumina particles and silica particles, the alumina particles that are embedded in the substrate are removed to some degree. However, as long as a polishing composition including alumina particles is used, there still remains the possibility that the alumina particles contained in the polishing composition are embedded in the substrate. In addition, since the proposed polishing compositions include both alumina particles and silica particles, there is caused a problem that the respective properties possessed by the alumina particles and by the silica particles are negated with each other, leading to worsening of the polishing rate and surface smoothness.

For these reasons, there have been proposed methods in which polishing is performed, with no use of alumina particles, using only silica particles. According to other conventional art, methods are proposed to combine colloidal silica and a polishing accelerator. According to other conventional art, methods are proposed in which polishing is performed using colloidal silica, as well as fumed silica, surface modified silica, silica produced in water glass process, and other types of silica, in particular colloidal silica having particular shapes. However, these methods do not achieve a sufficient polishing rate, and are required to make improvements. According to other conventional art, a method is proposed in which colloidal silica and fumed silica are used in combination. However, while this method results in an improvement in the polishing rate, the fumed silica has very low bulk density, leading to significant worsening of workability, such as in the preparation of slurries, and also having concerns about health due to its dusts. According to other conventional art, a method is proposed in which crushed silica particles are used, thereby to obtain a polishing rate approximate to that obtained when alumina particles are used. However, this method has a problem of worsening the surface smoothness, and is required to make improvements.

These methods in which polishing is performed using only silica particles present a problem that there is a worse roll-off, relative to when alumina particles are used. Various additives have been studied for reducing the roll-off. For example, there were proposed to add, for example, nonionic surfactants, cellulose derivatives, and alkylbenzenesulfonate surfactants and polyoxyethylene sulfonic acid ester surfactants. However, while these additives result in a reducing the roll-off they do not provide a sufficient effect of reducing the roll-off and also have a disadvantage that the polishing rate is significantly decreased, depending on the compatibility with the silica particles used.

SUMMARY

Embodiments of the invention have been made in light of the problems caused by these conventional techniques, and has an object to provide a polishing composition making it possible to achieve a high polishing rate and at the same time, to obtain a good surface smoothness and a reducing the roll-off, without the use of alumina particles.

Embodiments of the invention demonstrate that by combining colloidal silica and wet-process silica particles formed through a pulverization step and by adding a particular water-soluble polymer compound, it is possible to achieve a higher polishing rate and a better surface smoothness and further a greater reducing the roll-off than expected when particles of the respective types of silica are used alone.

Therefore, according to various embodiment of the invention, there is provided is a polishing composition as described below.

According to at least one embodiment, polishing composition including colloidal silica, pulverized wet-process silica particles, and a water-soluble polymer compound, wherein the water-soluble polymer compound is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid.

According to at least one embodiment, the colloidal silica has an average particle size of 5 to 200 nm and the wet-process silica particles have an average particle size of 0.1 to 1.0 μm.

According to at least one embodiment, a value of the ratio of the average particle size of the wet-process silica particles to that of the colloidal silica is from 2.0 to 30.0.

According to at least one embodiment, a total concentration of the colloidal silica and the wet-process silica particles is 1 to 50 mass %, wherein the colloidal silica accounts for 5 to 95 mass % and the wet-process silica particles account for 5 to 95 mass %, of the whole of the colloidal silica and the wet-process silica particles.

According to at least one embodiment, the water-soluble polymer compound is a copolymer further having a constituent unit derived from a hydrophobic monomer.

According to at least one embodiment, the water-soluble polymer compound has an average molecular weight of 500 to 20000.

According to at least one embodiment, the content of the water-soluble polymer compound is 0.0001 to 1.0 mass %.

According to at least one embodiment, the composition is an aqueous composition further including an acid and an oxidizing agent, wherein a pH value is 0.1 to 4.0.

According to at least one embodiment, the composition is used for polishing an aluminum magnetic disk substrate that is plated with electroless nickel-phosphorus.

According to another embodiment, there is provided a method for polishing a magnetic disk substrate, wherein the magnetic disk substrate is polished using the polishing composition according to various embodiments of the invention.

Embodiments of the invention make it possible that in polishing the surface of an aluminum magnetic disk substrate for a magnetic recording medium on which a coating has been formed by electroless nickel-phosphorus plating on the surface of a substrate of an aluminum alloy, particles of two types of silica are used in combination, thereby achieving a higher polishing rate, while obtaining a better surface smoothness, relative to when particles of the respective types of silica are used alone. Further, the polishing composition of the present invention makes it possible to obtain a reducing the roll-off by adding a particular water-soluble polymer compound, together with two types of silica particles.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the invention are better understood with regard to the following Detailed Description, appended Claims, and accompanying FIGURE. It is to be noted, however, that the FIGURE illustrates only various embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it may include other effective embodiments as well.

FIG. 1 represents a cross-sectional view of a substrate for providing an explanation on the measurement of the roll-off when the surface of the substrate has been polished according to an embodiment of the invention.

DETAILED DESCRIPTION

Advantages and features of the present invention and methods of accomplishing the same will be apparent by referring to embodiments described below in detail in connection with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various different forms. The embodiments are provided only for completing the disclosure of the present invention and for fully representing the scope of the present invention to those skilled in the art.

For simplicity and clarity of illustration, the drawing FIGURE illustrates the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the discussion of the described embodiments of the invention. Additionally, elements in the drawing FIGURE is not necessarily drawn to scale. For example, the dimensions of some of the elements in the FIGURE may be exaggerated relative to other elements to help improve understanding of embodiments of the invention. Like reference numerals refer to like elements throughout the specification.

Embodiments of the invention will be described below, but the invention is not limited to the embodiments described below, and it should be understood that the scope of the present invention includes various embodiments in which the embodiments described below are modified, improved, or changed as appropriate, based on the ordinary knowledge of those skilled in the art, within the scope not deviating from the spirit of the present invention.

1. Polishing Compositions

A polishing composition, according to at least one embodiment, is an aqueous composition including at least, colloidal silica, wet-process silica particles, and a water-soluble polymer compound. The water-soluble polymer compound is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid. The colloidal silica preferably has an average particle size of 5 to 200 nm, and the wet-process silica particles preferably have an average particle size of 0.1 to 1.0 μm. The value of the ratio (B/A) of the average particle size of the wet-process silica particles (B) to that of the colloidal silica (A) preferably is from 2.0 to 30.0. Here, the wet-process silica particles are cracked silica obtained by pulverization in their production process. Thus, the process for producing the wet-process silica particles includes a pulverization step.

A polishing composition, according to at least one embodiment, contains colloidal silica and a certain type of wet-process silica particles, thereby achieving a high polishing rate and at the same time, a good surface smoothness, which are beyond expectation, compared to when particles of the respective types of silica are used alone. In general, when particles of large particle sizes (mainly the wet-process silica particles according to at least one embodiment) and particles of small particle sizes (mainly the colloidal silica according to at least one embodiment) are used in combination in the polishing composition, there is a tendency that the polishing rate and surface smoothness achieved by using such polishing composition are dominated by those provided by the particles of large particle sizes. This means that in general, the polishing rate achieved by using the polishing composition including particles of large particle sizes and particles of small particle sizes does not significantly exceed the rate provided by the particles of large particle sizes, and the surface smoothness achieved by using the polishing composition is equal to the smoothness provided by the particles of large particle sizes and the smoothness is inferior to that of provided by the particles of small particle sizes. In contrast, the polishing composition, according to at least one embodiment, has a significantly higher polishing rate, relative to when either the colloidal silica or the wet-process silica particles are used alone, and retains a good surface smoothness. It can be said that these effects are remarkable in that they cannot be expected from conventional, common general technical knowledge.

The polishing composition, according to at least one embodiment, contains a water-soluble polymer compound that is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid, in addition to the above-described colloidal silica and certain type of wet-process silica particles, thereby achieving a reducing the roll-off. The effect that the roll-off is reduced by the water-soluble polymer compound is exerted when it is used in combination with colloidal silica and a certain type of wet-process silica particles. That is, the effect is not exerted when the water-soluble polymer compound is used in combination with colloidal silica alone, or with wet-process silica particles alone.

The polishing composition, according to at least one embodiment, will be described below in more detail. In the following description, when there is described simply as a “polishing composition,” it refers to the polishing composition according to at least one embodiment, unless otherwise specified. Also in the following description, when there is described simply as “colloidal silica” or “wet-process silica particles,” it refers to colloidal silica or wet-process silica particles for use in the present invention, respectively, unless otherwise specified.

1-1. Colloidal Silica

The colloidal silica that is contained in the polishing composition, according to at least one embodiment, preferably has an average particle size of 5 to 200 nm. Average particle sizes equal to or above 5 nm make it possible to prevent the polishing rate from decreasing. Average particle sizes of colloidal silica equal to or below 200 nm makes it possible to prevent the surface smoothness from worsening. The average particle size of colloidal silica is more preferably 5 to 150 nm, further preferably 30 to 100 nm. The average particle sizes in the present application refer to a median diameter (D50).

Colloidal silica is known in various shapes, for example, in spherical, chain-like, kompeito-typed (like particles having convexes on the surface), heteromorphic, and other shapes, and forms colloid in which primary silica particles are monodispersed in water. The colloidal silica for use in accordance with at least one embodiment particularly preferably is of a spherical or nearly spherical shape. The use of colloidal silica that is of a spherical or nearly spherical shape allows a further improvement in the surface smoothness. Colloidal silica is obtained, for example, by water glass process in which sodium or potassium silicates are used as a raw material, and by methods in which an alkoxysilane such as tetraethoxysilane is hydrolyzed with an acid or with an alkali.

1-2. Wet-Process Silica Particles

The wet-process silica particles for use in accordance with at least one embodiment refer to particles prepared from wet-process silica obtained as precipitated silicic acid by adding an aqueous alkali silicate solution and an inorganic acid or an aqueous solution of an inorganic acid into a reaction vessel, and do not include the colloidal silica described above.

The aqueous alkali silicate solution that is used as a raw material for wet-process silica is an aqueous sodium silicate solution, an aqueous potassium silicate solution, an aqueous lithium silicate solution, and others. In general, an aqueous sodium silicate solution is preferably used. The inorganic acid can be sulfuric acid, hydrochloric acid, nitric acid, and others. In general, sulfuric acid is preferably used. After the completion of the reaction, the reaction solution is filtered, and washed with water and then dried in a dryer so as to have a water content of 6% or lower. The dryer can be any of a stationary dryer, a spray dryer, and a fluidized dryer. Subsequently, the dried material is pulverized in a pulverizer such as a jet mill, followed by classification to obtain wet-process silica particles.

The wet-process silica particles having been cracked by pulverization in this way are of angular shapes and have a higher degree of polishing ability than that of nearly spherical particles.

According to at least one embodiment, the wet-process silica particles preferably have an average particle size of 0.1 to 1.0 μm, more preferably 0.2 to 1.0 μm, further preferably 0.2 to 0.6 μm. Average particle sizes of wet-process silica particles equal to or above 0.1 μm make it possible to prevent the polishing rate from decreasing. Average particle sizes of wet-process silica particles equal to or below 1.0 μm make it possible to prevent the surface smoothness from worsening.

According to at least one embodiment, the value of the ratio (B/A) between the average particle sizes of the wet-process silica particles (B) and the colloidal silica (A) preferably is from 2.0 to 30.0, more preferably from 2.0 to 16.0, further preferably from 3.0 to 16.0, particularly preferably from 3.0 to 10.0, most preferably from 4.0 to 10.0. Ratios of the average particle sizes equal to or above 2.0 make it possible to improve the polishing rate. Ratios of the average particle sizes equal to or below 30.0 make it possible to prevent the surface smoothness from worsening.

According to at least one embodiment, the total concentration of the colloidal silica and the wet-process silica particles is preferably 1 to 50 mass %, more preferably 2 to 40 mass %, in the whole polishing composition. Total concentrations of the two types of silica particles equal to or above 1 mass % make it possible to prevent the polishing rate from decreasing. Total concentrations of the two types of silica particles equal to or below 50 mass % make it possible to retain a sufficient polishing rate without using excessive silica particles.

According to at least one embodiment, the colloidal silica preferably accounts for 5 to 95 mass %, more preferably 20 to 80 mass %, of the whole of the silica particles. Percentages of the colloidal silica equal to or above 5 mass % make it possible to prevent the surface smoothness from worsening. Percentages of the colloidal silica equal to or below 95 mass % make it possible to prevent the polishing rate from decreasing.

According to at least one embodiment, the wet-process silica particles preferably account for 5 to 95 mass %, more preferably 20 to 80 mass/%, of the whole of the silica particles. Percentages of the wet-process silica particles equal to or below 95 mass % make it possible to prevent the surface smoothness from worsening. Percentages of the wet-process silica particles equal to or above 5 mass % make it possible to prevent the polishing rate from decreasing.

1-3. Water-Soluble Polymer Compounds

According to at least one embodiment, the water-soluble polymer compound is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid. Accordingly, the water-soluble polymer compound is a polymer compound resulting from polymerization using an unsaturated aliphatic carboxylic acid and/or a salt thereof as a monomer, which has as an essential constituent unit, the constituent unit derived from the unsaturated aliphatic carboxylic acid. Examples of the unsaturated aliphatic carboxylic acid and/or a salt thereof include, for example, acrylic acid, methacrylic acid, maleic acid, itaconic acid, and salts thereof.

According to at least one embodiment, the constituent unit derived from the unsaturated aliphatic carboxylic acid may be at least partially contained as its carboxylate salt in the water-soluble polymer compound. Examples of the carboxylate salt include, for example, sodium, potassium, magnesium, ammonium, amine, and alkylammonium salts.

In order that the constituent unit derived from an unsaturated aliphatic carboxylic acid is contained as a carboxylic acid in the water-soluble polymer compound, polymerization may be performed using the unsaturated aliphatic carboxylic acid as a monomer, or alternatively using a salt of the unsaturated aliphatic carboxylic acid as a monomer, followed by cation exchange to convert the carboxylate salt to the carboxylic acid. On the other hand, in order that the constituent unit derived from an unsaturated aliphatic carboxylic acid is contained as a carboxylate salt in the water-soluble polymer compound, polymerization may be performed using a salt of the unsaturated aliphatic carboxylic acid as a monomer, or alternatively using the unsaturated aliphatic carboxylic acid as a monomer, followed by neutralization with a base to convert the carboxylic acid to the carboxylate salt.

To assess the ratio of the constituent units contained as carboxylate salt and as carboxylic acid in the water-soluble polymer compound, use can be made of the pH value of an aqueous solution of the water-soluble polymer compound. When the pH value of the aqueous solution of the water-soluble polymer compound is low, it can be assessed that the water-soluble polymer compound has a high content of the constituent unit contained as carboxylic acid. When the pH value of the aqueous solution of the water-soluble polymer compound is high, on the other hand, it can be assessed that the water-soluble polymer compound has a high content of the constituent unit contained as carboxylate salt. In the present invention, it is possible to use a water-soluble polymer compound giving, for example, the pH value in the range of 1 to 13 in the aqueous solution thereof having a concentration of 10 mass %.

In cases where the water-soluble polymer compound is a copolymer having the constituent unit derived from the unsaturated aliphatic carboxylic acid, the monomer that is used for copolymerization in combination with the unsaturated aliphatic carboxylic acid and/or a salt thereof is, for example, a hydrophobic monomer. Examples of the hydrophobic monomer include, for example, unsaturated aliphatic carboxylic acid esters such as acrylic esters, methacrylic esters, maleic esters, and itaconic esters; and aromatic monomers such as styrene, α-methylstyrene, divinylbenzene, and N-phenylmaleimide. Preferable examples of the hydrophobic monomer include, for example, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate, t-butyl methacrylate, pentyl acrylate, pentyl methacrylate, hexyl acrylate, hexyl methacrylate, heptyl acrylate, heptyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate.

It is preferable that a polymer or copolymer is formed by polymerization using these monomer components alone or in combination. As combinations of monomers for copolymerization, there are preferably used combinations of acrylic acid and/or a salt thereof and an acrylic ester, of acrylic acid and/or a salt thereof and a methacrylic ester, and of methacrylic acid and/or a salt thereof and an acrylic ester, of methacrylic acid and/or a salt thereof and a methacrylic ester.

According to at least one embodiment, the water-soluble polymer compound preferably has a weight average molecular weight of 500 or more and 20000 or less, more preferably 1000 or more and 10000 or less. The weight average molecular weight of the water-soluble polymer compound is determined by gel permeation chromatography (GPC) in polyacrylic acid equivalent molecular weight.

According to at least one embodiment, the concentration of the water-soluble polymer compound in the polishing composition preferably is 0.0001 mass % or more and 1.0 mass % or less, more preferably 0.001 mass % or more and 0.5 mass % or less, further preferably 0.005 mass % or more and 0.2 mass % or less, particularly preferably 0.01 mass % or more and 0.1 mass % or less, in terms of solids content.

1-4. Other Components

In addition to the colloidal silica and the wet-process silica particles, the polishing composition can include other particles, as particles contained in the polishing composition. In this case, it is particularly preferable that the polishing composition does not include alumina particles from the viewpoint of reducing embedding of them in a substrate to be polished.

According to at least one embodiment, the polishing composition preferably includes an acid, in addition to the silica particles and the water-soluble polymer compound. The acid may be an acid and/or a salt thereof. Specific examples of the acid include inorganic acids such as nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, phosphonic acid, phosphinic acid, pyrophosphoric acid, and tripolyphosphoric acid; organic phosphonic acids such as 2-aminoethylphosphonic acid, 1-hydroxyethyliden-1,1-diphosphonic acid, aminotri (methylenephosphonic acid), ethylene diamine tetra(methylenephosphonic acid), diethylene triamine (methylenephosphonic acid), ethane-1,1-diphosphonic acid, and methane hydroxyphosphonic acid; aminocarboxylic acids such as glutamic acid and aspartic acid; and carboxylic acids such as citric acid, tartaric acid, oxalic acid, nitroacetic acid, and maleic acid. Counterions in cases of using these acids are not limited in particular, and specific examples of these counterions include, for example, metal, and ammonium and alkylammonium ions.

According to at least one embodiment, the polishing composition preferably includes an oxidizing agent, in addition to the silica particles and the water-soluble polymer compound. The oxidizing agent includes, for example, peroxides, permanganic acid and salts thereof, and chromic acid and salts thereof. Specific examples of the oxidizing agent include, for example, hydrogen peroxide, sodium peroxide, barium peroxide, and potassium permanganate. Among them, preference is given to hydrogen peroxide.

According to at least one embodiment, the polishing composition may include fungicides, microbicides, and others, besides the above-described components.

1-5. Physical Properties

According to at least one embodiment, the polishing composition preferably has a pH value of 0.1 to 4.0, more preferably 0.5 to 3.0. The pH values of the polishing composition equal to or above 0.1 make it possible to prevent the surface smoothness from worsening. The pH values of the polishing composition equal to or below 4.0 make it possible to prevent the polishing rate from decreasing.

According to at least one embodiment, the polishing composition can be used for polishing various electronic components, such as semiconductors and magnetic recording media including hard disks. In particular, the polishing composition is suitably employed for polishing an aluminum magnetic disk substrate of an aluminum alloy. Further suitably, the polishing composition can be employed for polishing an aluminum magnetic disk substrate which is plated with electroless nickel-phosphorus. The electroless nickel-phosphorus plating is usually performed under conditions where the pH is 4 to 6. Conditions with the pH of not higher than 4 have a tendency for nickel to dissolve, and thus it is difficult that the plating takes place. In polishing, on the other hand, nickel has a tendency toward dissolution, for example, under conditions with the pH of not higher than 4.0, and thus the use of the polishing composition of the present invention can result in an increase in the polishing rate.

2. Methods for Polishing Magnetic Disk Substrates

According to at least one embodiment, the polishing composition is suitable for polishing magnetic disk substrates, such as an aluminum magnetic disk substrate which has been subjected to electroless nickel-phosphorus plating (hereinafter referred to as an aluminum disk), and a glass magnetic disk substrate. In particular, the polishing composition is suitable for use in polishing the aluminum disk. Therefore, the present invention is directed to a method for polishing the magnetic disk substrate wherein the magnetic disk substrate is polished using the above-described polishing composition. The polishing composition can also be used for polishing semiconductor substrates, for example, of silicon carbide, silicon, germanium, gallium arsenide, gallium phosphide, indium phosphide, and gallium nitride; single crystal substrates, for example, of sapphire, lithium tantalate, and lithium niobate; magnetic heads, and others.

According to at least one embodiment, polishing methods in which the polishing composition in accordance with at least one embodiment can be applied include, for example, a method (called polishing) in which a polishing pad is attached to a surface plate of a polishing machine, a polishing composition is fed onto a surface to be polished of a polishing object (for example, an aluminum disk) or onto the polishing pad, and the surface to be polished is rubbed with the polishing pad. For example, when the front- and back-side surfaces of an aluminum disk are polished at a time, a method is used in which use is made of a double side polishing machine provided with an upper surface plate and a lower surface plate to each of which a polishing pad has been attached. In this method, the front- and back-side surfaces of the aluminum disk are polished by sandwiching the aluminum disk between the polishing pads which each have been attached to the upper and lower surface plates, feeding the polishing composition between the surface to be polished and the polishing pad, and concurrently rotating these two polishing pads.

For the polishing pad, use can be made of ones of polyurethane type, suede type, non-woven fabric type, and any other type.

EXAMPLES

Embodiments of the invention will be described below in a specific manner based on Examples which follow, but it goes without saying that these Examples are not intended to limit the present invention, which can be carried out in various embodiments, as long as they fall within the technical scope of the present invention.

(1) Methods for Preparing Polishing Compositions Examples 1 to 21 and Comparative Examples 1 to 20

The polishing compositions used in Examples 1 to 21 and Comparative Examples 1 to 20 were ones including the following materials in the following contents or added amounts.

[Colloidal silica 1] (commercially available colloidal silica, having an average particle size (D50) of 51 nm on a cumulative volume basis). Its contents are indicated in Tables 1 to 4. The colloidal silica was not contained in Comparative Examples 2, 9, 10, and 16.

[Colloidal silica 2] (commercially available colloidal silica, having an average particle size (D50) of 112 nm on a cumulative volume basis). Its contents are indicated in Table 3. The colloidal silica was not contained in Examples 1 to 16 and 18 to 21 and in Comparative Examples 1 to 20.

[Wet-process silica 1] (commercially available wet-process silica particles, having an average particle size (D50) of 0.3 μm). Its contents are indicated in Tables 1, 3, and 4. The wet-process silica was not contained in Examples 6 to 10 and in Comparative Examples 1, 3, and 10 to 16.

[Wet-process silica 2] (commercially available wet-process silica particles, having an average particle size (D50) of 0.4 μm). Its contents are indicated in Table 2. The wet-process silica was not contained in Examples 1 to 5 and 11 to 21 and in Comparative Examples 1 to 9 and 17 to 20.

[Sulfuric acid] 1.34 mass % (adjustment was made to this concentration in all of Examples 1 to 21 and Comparative Examples 1 to 20.)

[Hydrogen peroxide] 0.9 mass % (adjustment was made to this concentration in all of Examples 1 to 21 and Comparative Examples 1 to 20.)

[Ammonia] 0.22 mass % (adjustment was made to this concentration in all of Examples 1 to 21 and Comparative Examples 1 to 20.)

[Acrylic polymer 1] (e.g., M-8, manufactured by Toagosei Co., Ltd. and having a weight average molecular weight of 6000). Its added amounts are indicated in Tables 1 to 4. The acrylic polymer was not contained in Examples 15 and 16 and in Comparative Examples 3 to 9 and 11 to 20.

A 10 mass % aqueous solution of acrylic polymer 1 usually has a pH value of 3.4. In Examples 1 to 14 and 17 and Comparative Examples 1, 2, and 10, use was made of this usual solution of acrylic polymer 1.

In addition, an aqueous solution of sodium hydroxide was added to such an aqueous solution of acrylic polymer 1 to prepare 10 mass % aqueous solutions of acrylic polymer 1 which were adjusted to have respective pH values of 5.3, 7.6, 8.9, and 10.8. As acrylic polymer 1, Example 18 used a 10 mass % solution having a pH value of 5.3, Example 19 used a 10 mass % solution having a pH value of 7.6, Example 20 used a 10 mass % solution having a pH value of 8.9, and Example 21 used a 10 mass % solution having a pH value of 10.8.

[Acrylic polymer 2] (e.g., CM-15, manufactured by Toagosei Co., Ltd. and having a weight average molecular weight of 4500). Its contents are indicated in Table 3. The acrylic polymer was not contained in Examples 1 to 14 and 17 to 21 and in Comparative Examples 1 to 20.

A 10 mass % aqueous solution of acrylic polymer 2 usually has a pH value of 3.3. In Examples 15 and 16, use was made of this usual solution of acrylic polymer 2.

[Sodium lauryl sulfate] (e.g., ARUSUKOPU LS-30, manufactured by TOHO Chemical Industry Co., Ltd. and referred to as “LS-30” in Table 4), 0.02 mass %. Use was made in Comparative Examples 17 and 19.

[Polyoxyethylene tridecyl ether sulfuric acid sodium salt] (e.g., HITENOL 330T, manufactured by Dai-ich Kogyo Seiyaku Co., Ltd. and referred to as “330T” in Table 4). 0.02 mass %. Use was made in Comparative Examples 18 and 20.

Example 17 used silica particles of three types: colloidal silica 1, colloidal silica 2, and wet-process silica 1. Their contents in the polishing composition are indicated in Table 3; the ratio of the contents of the colloidal silicas 1 and 2 was 64:22, and the overall average particle size (D50) of the whole of the colloidal silicas 1 and 2 was 63 nm on a cumulative volume basis.

The particle diameter (e.g., Heywood diameter) of colloidal silica was determined as a Heywood diameter (projected area equivalent circular diameter) by taking photographs of fields of view at a magnification of ×100000 under a transmission electron microscope (TEM) (e.g., transmission electron microscope JEM 2000FX (200 kV), manufactured by JEOL Ltd.) and analyzing these photographs with an analysis software (e.g., Mac-View Ver. 4.0, manufactured by Mountech CO., Ltd.). The average particle size of colloidal silica is an average particle size (D50) when the particle diameters of about 2000 particles of colloidal silica were analyzed as described above and the particle size at which the cumulative particle size distribution (on a cumulative volume basis) from a smaller particle size reaches 50% is calculated using an above-mentioned analysis software (e.g., Mac-View Ver. 4.0, manufactured by Mountech CO., Ltd.).

The average particle size of wet-process silica particles was determined using a dynamic light scattering particle size distribution analyzer (e.g., Microtrac UPA, manufactured by NIKKISO CO., Ltd.) for particles having particle sizes of not more than 0.4 μm, and a laser diffraction particle size distribution analyzer (e.g., SALD 2200, manufactured by SHIMADZU CORPORATION) for particles having particle sizes of more than 0.4 μm. The average particle size of wet-process silica particles is an average particle size (D50) at which the cumulative particle size distribution on a volume basis a smaller particle size reaches 50%.

Comparative Examples 21 to 24

The polishing compositions used in Comparative Examples 21 to 24 were ones including the following materials in the following contents or added amounts.

[Alumina] (commercially available alumina, having an average particle size (D50) of 0.4 μm). Its contents are indicated in Table 5.

[Sulfuric acid] 0.91 mass % (adjustment was made to this concentration in all of Comparative Examples 21 to 24.)

[HEDP (1-hydroxyethylidene-1,1-diphosphonic acid)] 0.61 mass % (adjustment was made to this concentration in all of Comparative Examples 21 to 24.)

[Hydrogen peroxide] 0.46 mass % (adjustment was made to this concentration in all of Comparative Examples 21 to 24.)

[Ammonia] 0.10 mass % (adjustment was made to this concentration in all of Comparative Examples 21 to 24.)

[Acrylic polymer 1] (e.g., CM-8, manufactured by Toagosei Co., Ltd. and having a weight average molecular weight of 6000). Its added amounts are indicated in Table 5. The acrylic polymer was not contained in Comparative Examples 21 and 24.

A 10 mass % aqueous solution of acrylic polymer 1 usually has a pH value of 3.4. In Comparative Examples 22 and 23, use was made of this usual solution of acrylic polymer 1.

[Polyoxyethylene tridecyl ether sulfuric acid sodium salt] (e.g., HITENOL 330T, manufactured by Dai-ich Kogyo Seiyaku Co., Ltd. and referred to as “330T” in Table 5). 0.06 mass %. Use was made in Comparative Example 24.

The average particle size of alumina particles was determined using a laser diffraction particle size distribution analyzer (e.g., SALD 2200, manufactured by SHIMADZU CORPORATION). The average particle size of alumina particles is an average particle size (D50) at which the cumulative particle size distribution on a volume basis from a smaller particle size reaches 50/a %.

(2) Polishing Conditions

Aluminum disks with an outer diameter of 95 mm that is plated with electroless nickel-phosphorus were used as a substrate to be polished, and polishing was performed under the following polishing conditions.

Polishing machine: 9B double-side polishing machine, manufactured, for example, by System Seiko Co., Ltd.

Polishing pad: P1 pads, manufactured, for example, by FILWEL Co., Ltd.

Surface plate rotating speed: upper surface plate: −13.0 min⁻¹

Lower surface plate: 16.0 min⁻¹

Feed of polishing composition: 70 ml/min

Polishing time: polishing was performed up to the time when the amount of polishing reached 1.2 to 1.5 μm per one side. (240 to 720 seconds)

Processing pressure: 120 kPa (for Examples 1 to 21 and Comparative Examples 1 to 20) and 80 kPa (for Comparative Examples 21 to 24)

(3) Assessment of Polished Disk Surfaces (3-1) Polishing Rate Ratios

The polishing rate was calculated by measuring the decreased mass of a polished aluminum disk, according to the following formula: Polishing rate (μm/min)=decreased mass of a polished aluminum disk (g)/polishing time (min)/area of one side of the aluminum disk (cm²)/density of the coating by electroless nickel-phosphorus plating (g/cm³)/2×10⁴, wherein the area of one side of the aluminum disk was 65.9 cm² and the density of the coating by electroless nickel-phosphorus plating was 8.0 g/cm³.

The polishing rate ratio is a relative value when the polishing rate for a Comparative Example which was determined using the above formula is 1 (as a reference). For Examples 1 to 21 and Comparative Examples 1 to 20, the polishing rate for Comparative Example 3 was set to be 1 (as a reference); for Comparative Examples 21 to 24, the polishing rate for Comparative Example 21 was set to be 1 (as a reference). The polishing rate for Comparative Example 3 was 0.111 μm/min, and for Comparative Example 21 was 0.325 μm/min.

(3-2) Pits

Pits were measured under a three-dimensional surface structure analyzing microscope using scanning white-light interferometry, manufactured by Zygo Corp. The measurement was made using a measurement apparatus, manufactured by Zygo Corp. (e.g., New View 5032 with a lens of an ×2.5 magnification and a zoom of 0.5 times) and a analysis software, manufactured by Zygo Corp. (Metro Pro). The observed shape profiles were evaluated as “good” for profiles having few pits, “fair” for profiles having some pits, and “failure” for profiles having many pits. In the case of profiles rated as “failure,” they were visually observed.

(3-3) Surface Roughness (Zygo-Ra)

The surface roughness of aluminum disks (Ra) was determined under a three-dimensional surface structure analyzing microscope using scanning white-light interferometry, manufactured by Zygo Corp. (hereinafter, the surface roughness measured by this method is referred to as a “Zygo-Ra”). The determination was made under conditions where a measurement apparatus, manufactured by Zygo Corp. (e.g., New View 5032 with a lens of a ×2.5 magnification and a zoom of 0.5 times) and a analysis software, manufactured by Zygo Corp. (Metro Pro) were used, the filter had an FFT Fixed Pass wavelength set to be 0.00 to 0.08 mm, and the measurement area was 5.68 mm×4.26 mm. In the tables, the surface roughness being rated as “not measurable” indicates a state where there are pits and the surface roughness cannot be determined by the above-described method.

(3-4) Roff-Off Ratios

As an assessment of the edge shape of a polished substrate, measurement was made of the roll-off, by which the degree of edge rounding is quantified. The roll-off was measured using a measurement apparatus, manufactured by Zygo Corp. (e.g., New View 5032 with a lens of an ×2.5 magnification and a zoom of 0.5 times) and a analysis software, manufactured by Zygo Corp. (Metro Pro).

With reference to FIG. 1, an explanation is given of a method for measuring the roll-off. FIG. 1 shows a cross-sectional view perpendicular to a polished surface of an aluminum disk having an outer diameter of 95 mm and, a subject to be polished, which has been subjected to electroless nickel-phosphorus plating, taken as a plane through the center of the disk. When the roll-off is measured, a vertical line h is first drawn at the edge of circumference of the polished disk, and a line j is drawn in parallel with the line h and at a distance of 4.50 mm from the line h to the center of the polished surface of the disk, and the position at which the line of the cross section of the disk intersects the line j is defined as point A. Also, a line k is drawn in parallel with the line h and at a distance of 0.50 mm from the line h, and the position at which the line of the cross section of the disk intersects the line k is defined as point B. A line m is drawn connecting between points A and B, a line t is further drawn perpendicular to the line m, and the intersection of the (contour) line of the cross section of the disk and the line t is defined as point C and the intersection of the line m and the line t is defined as point D. The largest distance between points C and D is determined as a value for the roll-off.

The roll-off ratio is a relative value when the roll-off for a Comparative Example which was determined using the above method is 1 (as a reference). For Examples 1 to 21 and Comparative Examples 1 to 20, the roll-off value for Comparative Example 3 was set to be 1 (as a reference); for Comparative Examples 21 to 24, the roll-off value for Comparative Example 21 was set to be 1 (as a reference). The roll-off value for Comparative Example 3 was 83.5 nm, and for Comparative Example 21 was 11.1 nm. Tables 1 to 4 show relative roll-off values between Comparative Examples and Examples which had the same composition of silica particles but “did not contain a water-soluble polymer compound” (Comparative Examples) and “contained a water-soluble polymer compound” (Examples), which are values of the ratio of roll-off values for Examples to those for their corresponding Comparative Examples. In Tables 1 and 2, the roll-off ratio being presented as “not measurable” indicates a state where there are many pits and the roll-off cannot be measured by the above-described method.

TABLE 1 Colloidal Water-soluble polymer Colloidal silica Wet-process silica silica:wet- compound Average Average process 10% particle particle Total silica Particle silica aqueous Added size size concentration size particles solution amount Type D50 (nm) Type D50 (μm) (mass %) ratio (mass ratio) Type pH (mass %) Comparative Colloidal 51 none — 4.4 5.9 10:0  Acrylic 3.4 0.02 Example 1 silica 1 polymer 1 Example 1 Colloidal 51 Wet-process 0.3 4.4 9:1 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 2 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 3 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 4 Colloidal 51 Wet-process 0.3 4.4 3:7 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 5 Colloidal 51 Wet-process 0.3 4.4 1:9 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Comparative none — Wet-process 0.3 4.4  0:10 Acrylic 3.4 0.02 Example 2 silica 1 polymer 1 Comparative Colloidal 51 none — 4.4 5.9 10:0  none — — Example 3 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 9:1 none — — Example 4 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 7:3 none — — Example 5 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 5:5 none — — Example 6 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4 4 3:7 none — — Example 7 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 1:9 none — — Example 8 silica 1 silica 1 Comparative none — Wet-process 0.3 4.4  0:10 none — — Example 9 silica 1 Roll-off value (vs composition Polishing Surface Roll-off not containing a water-soluble rate ratio (vs roughness value (vs polymer compound) Comparative Zygo-Ra Comparative Roll-off pH Example 3) Pits (Å) Example 3) value Compared to Comparative 1.2 0.92 good 1.32 1.04 1.04 Comparative Example 1 Example 3 Example 1 1.2 1.31 good 1.56 1.01 0.94 Comparative Example 4 Example 2 1.2 1.65 good 1.75 0.90 0.83 Comparative Example 5 Example 3 1.2 2.00 good 1.95 0.57 0.64 Comparative Example 6 Example 4 1.2 2.33 good 1.98 0.40 0.56 Comparative Example 7 Example 5 1.2 2.33 good 2.09 0.35 0.67 Comparative Example 8 Comparative 1.2 1.58 failure not not — Comparative Example 2 measurable measurable Example 9 Comparative 1.2 1 good 1.37 1   1.00 Comparative Example 3 Example 3 Comparative 1.2 1.22 good 1.52 1.08 1.00 Comparative Example 4 Example 4 Comparative 1.2 1.43 good 1.74 1.09 1.00 Comparative Example 5 Example 5 Comparative 1.2 1.65 good 1.95 0.89 1.00 Comparative Example 6 Example 6 Comparative 1.2 1.94 good 2.00 0.71 1.00 Comparative Example 7 Example 7 Comparative 1.2 1.98 fair 2.23 0.52 1.00 Comparative Example 8 Example 8 Comparative 1.2 1.49 failure not not — Comparative Example 9 measurable measurable Example 9

TABLE 2 Colloidal Water-soluble polymer Colloidal silica Wet-process silica silica:wet- compound Average Average process 10% particle particle Total silica Particle silica aqueous Added size size concentration size particles solution amount Type D50 (nm) Type D50 (μm) (mass %) ratio (mass ratio) Type pH (mass %) Comparative Colloidal 51 none — 4.4 7.8 10:0  Acrylic 3.4 0.02 Example 1 silica 1 polymer 1 Example 6 Colloidal 51 Wet-process 0.4 4.4 9:1 Acrylic 3.4 0.02 silica 1 silica 2 polymer 1 Example 7 Colloidal 51 Wet-process 0.4 4.4 7:3 Acrylic 3.4 0.02 silica 1 silica 2 polymer 1 Example 8 Colloidal 51 Wet-process 0.4 4.4 5:5 Acrylic 3.4 0.02 silica 1 silica 2 polymer 1 Example 9 Colloidal 51 Wet-process 0.4 4.4 3:7 Acrylic 3.4 0.02 silica 1 silica 2 polymer 1 Example 10 Colloidal 51 Wet-process 0.4 4.4 1:9 Acrylic 3.4 0.02 silica 1 silica 2 polymer 1 Comparative none — Wet-process 0.4 4.4  0:10 Acrylic 3.4 0.02 Example 10 silica 2 polymer 1 Comparative Colloidal 51 none — 4.4 7.8 10:0  none — — Example 3 silica 1 Comparative Colloidal 51 Wet-process 0.4 4.4 9:1 none — — Example 11 silica 1 silica 2 Comparative Colloidal 51 Wet-process 0.4 4.4 7:3 none — — Example 12 silica 1 silica 2 Comparative Colloidal 51 Wet-process 0.4 4.4 5:5 none — — Example 13 silica 1 silica 2 Comparative Colloidal 51 Wet-process 0.4 4.4 3:7 none — — Example 14 silica 1 silica 2 Comparative Colloidal 51 Wet-process 0.4 4.4 1:9 none — — Example 15 silica 1 silica 2 Comparative none — Wet-process 0.4 4.4  0:10 none — — Example 16 silica 2 Roll-off value (vs composition Polishing Surface Roll-off not containing a water-soluble rate ratio (vs roughness value (vs polymer compound) Comparative Zygo-Ra Comparative Roll-off pH Example 3) Pits (Å) Example 3) value Compared to Comparative 1.2 0.92 good 1.32 1.04 1.04 Comparative Example 1 Example 3 Example 6 1.2 1.44 good 2.20 1.49 0.92 Comparative Example 11 Example 7 1.2 2.05 good 2.69 1.42 0.83 Comparative Example 12 Example 8 1.2 2.59 good 3.11 1.09 0.69 Comparative Example 13 Example 9 1.2 2.92 good 3.57 0.68 0.59 Comparative Example 14 Example 10 1.2 2.75 good 4.55 0.60 0.63 Comparative Example 15 Comparative 1.2 2.00 failure not not — Comparative Example 10 measurable measurable Example 16 Comparative 1.2 1 good 1.37 1   1.00 Comparative Example 3 Example 3 Comparative 1.2 1.35 good 2.18 1.62 1.00 Comparative Example 11 Example 11 Comparative 1.2 1.78 good 2.74 1.72 1.00 Comparative Example 12 Example 12 Comparative 1.2 2.14 good 3.08 1.57 1.00 Comparative Example 13 Example 13 Comparative 1.2 2.43 good 3.52 1.16 1.00 Comparative Example 14 Example 14 Comparative 1.2 2.33 good 4.63 0.95 1.00 Comparative Example 15 Example 15 Comparative 1.2 1.89 failure not not — Comparative Example 16 measurable measurable Example 16

TABLE 3 Colloidal Water-soluble polymer Colloidal silica Wet-process silica silica:wet- compound Average Average process 10% particle particle Total silica Particle silica aqueous Added size size concentration size particles solution amount Type D50 (nm) Type D50 (μm) (mass %) ratio (mass ratio) Type pH (mass %) Comparative Colloidal 51 Wet-process 0.3 4.4 5.9 7:3 none — — Example 5 silica 1 silica 1 Example 11 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4  0.005 silica 1 silica 1 polymer 1 Example 12 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 0.01 silica 1 silica 1 polymer 1 Example 2 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 13 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 0.10 silica 1 silica 1 polymer 1 Example 14 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 1.00 silica 1 silica 1 polymer 1 Example 15 Colloidal 51 Wet-process 0.3 4.4 5.9 7:3 Acrylic 3.3 0.02 silica 1 silica 1 polymer 2 Example 16 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.3 0.10 silica 1 silica 1 polymer 2 Example 17 Colloidal  (51) 63 Wet-process 0.3 4.4 4.8 86:14 Acrylic 3.4 0.02 silica 1 64 silica 1 polymer 1 Colloidal (112) silica 2 22 Roll-off value (vs composition Polishing Surface Roll-off not containing a water-soluble rate ratio (vs roughness value (vs polymer compound) Comparative Zygo-Ra Comparative Roll-off pH Example 3) Pits (Å) Example 3) value Compared to Comparative 1.2 1.43 good 1.74 1.09 1.00 Comparative Example 5 Example 5 Example 11 1.2 1.57 good 1.75 1.06 0.97 Comparative Example 5 Example 12 1.2 1.63 good 1.74 0.99 0.91 Comparative Example 5 Example 2 1.2 1.65 good 1.75 0.90 0.83 Comparative Example 5 Example 13 1.2 1.65 good 1.73 0.84 0.77 Comparative Example 5 Example 14 1.2 1.55 fair 1.73 0.61 0.56 Comparative Example 5 Example 15 1.2 1.67 good 1.81 0.91 0.83 Comparative Example 5 Example 16 1.2 1.65 good 1.86 0.8 0.73 Comparative Example 5 Example 17 1.2 1.59 good 2.00 0.51 — —

TABLE 4 Colloidal Water-soluble polymer Colloidal silica Wet-process silica silica:wet- compound Average Average process 10% particle particle Total silica Particle silica aqueous Added size size concentration size particles solution amount Type D50 (nm) Type D50 (μm) (mass %) ratio (mass ratio) Type pH (mass %) Comparative Colloidal 51 Wet-process 0.3 4.4 5.9 5:5 none — — Example 6 silica 1 silica 1 Example 3 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Example 18 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 5.3 0.02 silica 1 silica 1 polymer 1 Example 19 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 7.6 0.02 silica 1 silica 1 polymer 1 Example 20 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 8.9 0.02 silica 1 silica 1 polymer 1 Example 21 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 10.8  0.02 silica 1 silica 1 polymer 1 Comparative Colloidal 51 Wet-process 0.3 4.4 5.9 7:3 none — — Example 5 silica 1 silica 1 Example 2 Colloidal 51 Wet-process 0.3 4.4 7:3 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Comparative Colloidal 51 Wet-process 0.3 4.4 7:3 LS-30 — 0.02 Example 17 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 7:3 330T — 0.02 Example 18 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 5.9 5:5 none — — Example 6 silica 1 silica 1 Example 3 Colloidal 51 Wet-process 0.3 4.4 5:5 Acrylic 3.4 0.02 silica 1 silica 1 polymer 1 Comparative Colloidal 51 Wet-process 0.3 4.4 5:5 LS-30 — 0.02 Example 19 silica 1 silica 1 Comparative Colloidal 51 Wet-process 0.3 4.4 5:5 330T — 0.02 Example 20 silica 1 silica 1 Roll-off value (vs composition Polishing Surface Roll-off not containing a water-soluble rate ratio (vs roughness value (vs polymer compound) Comparative Zygo-Ra Comparative Roll-off pH Example 3) Pits (Å) Example 3) value Compared to Comparative 1.2 1.65 good 1.95 0.89 1.00 Comparative Example 6 Example 6 Example 3 1.2 2.00 good 1.95 0.57 0.64 Comparative Example 6 Example 18 1.2 1.94 good 1.98 0.59 0.66 Comparative Example 6 Example 19 1.2 1.89 good 1.92 0.61 0.69 Comparative Example 6 Example 20 1.2 1.91 good 1.98 0.61 0.69 Comparative Example 6 Example 21 1.2 1.91 good 1.98 0.65 0.73 Comparative Example 6 Comparative 1.2 1.43 good 1.74 1.09 1.00 Comparative Example 5 Example 5 Example 2 1.2 1.65 good 1.75 0.90 0.83 Comparative Example 5 Comparative 1.2 1.67 fair 1.66 1.01 0.93 Comparative Example 17 Example 5 Comparative 1.2 1.65 fair 1.68 1.00 0.92 Comparative Example 18 Example 5 Comparative 1.2 1.65 good 1.95 0.89 1.00 Comparative Example 6 Example 6 Example 3 1.2 2.00 good 1.95 0.57 0.64 Comparative Example 6 Comparative 1.2 1.94 good 1.96 0.90 1.01 Comparative Example 19 Example 6 Comparative 1.2 2.04 good 1.95 0.70 0.79 Comparative Example 20 Example 6

TABLE 5 Alumina Water-soluble polymer compound Average Alumina 10% Polishing Surface particle concen- aqueous Added rate ratio (vs roughness Roll-off value size tration solution amount Comparative Zygo-Ra (vs Comparative Type D50 (μm) (mass %) Type pH (mass %) pH Example 21) Pits (Å) Example 21) Comparative Alumina 0.4 4.4 none — — 1.2 1 good 2.32 1 Example 21 Comparative Alumina 0.4 4.4 Acrylic 3.4 0.02 1.2 1.01 good 2.22 1.59 Example 22 polymer 1 Comparative Alumina 0.4 4.4 Acrylic 3.4 0.06 1.2 1.01 good 2.18 1.43 Example 23 polymer 1 Comparative Alumina 0.4 4.4 330T — 0.06 1.2 0.97 good 2.13 0.57 Example 24

(4) Discussion

From the results shown in Tables 1 to 5, it can be found that when the colloidal silica and the wet-process silica particles were used in combination as in Examples 1 to 21, the polishing rate was significantly increased relative to those expected when silica particles of the respective types were used alone. In this regard, it is generally expected that the polishing rate which is expected when either the colloidal silica or the wet-process silica particles are used alone varies with a correlation mainly with the content of the wet-process silica particles. On the contrary, the results of Example 1 to 5, for example, are remarkable ones that significantly exceed this expectation. This is thought to be because a mutually complementary relationship between the colloidal silica and the wet-process silica particles is brought about in terms of polishing performance and as a synergistic effect, improves the polishing rate. Specifically, it is thought that by using the wet-process silica particles and the colloidal silica in combination as in the present invention, the colloidal silica particles are attached onto the surface of the wet-process silica particles. Such wet-process silica particles having colloidal silica particles adhered on their surface is thought to have an improved polishing ability relative to when the wet-process silica particles are used alone.

In addition, the polishing compositions of Examples 1 to 21 also have an improved polishing performance in surface shape characteristics such as pits and surface roughness. Also in these cases, wet-process silica particles having colloidal silica particles adhered on their surface are thought to be generated, effects provided by the colloidal silica adhered onto the wet-process silica particles improve the polishing ability of the wet-process silica particles and at the same time, result in an improvement in the surface smoothness. As has been described above, it is thought that a mutually complementary relationship between both the colloidal silica and the wet-process silica particles is brought about also in terms of surface smoothness, and synergistic effects are exerted.

Further, the polishing composition of Examples 1 to 21 resulted in a specific decrease in the roll-off by means of using a water-soluble polymer compound in combination with colloidal silica and wet-process silica particles. It is also shown from the results of Examples 3 and 18 to 21 that the effect of decreasing the roll-off was observed even when the pH of a 10 mass % aqueous solution of the water-soluble polymer compound was changed from an acidic range to an alkalin range, that is, at least portion of the carboxylic groups of the constituent unit derived from the unsaturated aliphatic carboxylic acid contained in the Water-soluble polymer formed a salt.

A comparison of the results of Comparative Examples 1 and 3 shows that the effect of decreasing the roll-off is not observed when the water-soluble polymer compound was added to the colloidal silica. When referring to the results of Comparative Example 2, pits were generated when the water-soluble polymer compound was added to the wet-process silica particles, and thus there is not observed any effect of the water-soluble polymer compound. In a comparison of the results of Comparative Examples 22 and 23 and of Comparative Example 21, it turns out that the addition of the water-soluble polymer compound to the alumina particles resulted in a worsened, rather than decreased, roll-off. From what has been mentioned above, it is possible that the combination of three elements, colloidal silica, wet-process silica particles, and a water-soluble polymer compound, result in an improvement of three polishing properties, polishing rate, surface smoothness, and roll-off.

The polishing composition, according to various embodiments, can be used for polishing electronic components such as semiconductors and magnetic recording media including hard disks. Particularly, the polishing composition can be used for surface polishing of a substrate for a magnetic recording medium, such as a glass magnetic disk substrate and an aluminum magnetic disk substrate. Further, the polishing composition can be used for surface polishing of an aluminum magnetic disk substrate for a magnetic recording medium, the substrate having a coating formed by electroless nickel-phosphorus plating on the surface of a substrate of an aluminum alloy.

Although embodiments of the invention have been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of the embodiments of the invention. Accordingly, the scope of the embodiments of the invention should be determined by the following claims and their appropriate legal equivalents. 

1. A polishing composition, comprising: colloidal silica; pulverized wet-process silica particles; and a water-soluble polymer compound, wherein the water-soluble polymer compound is a polymer or copolymer having a constituent unit derived from an unsaturated aliphatic carboxylic acid.
 2. The polishing composition according to claim 1, wherein the colloidal silica has an average particle size of 5 to 200 nm, and the wet-process silica particles have an average particle size of 0.1 to 1.0 μm.
 3. The polishing composition according to claim 1, wherein a value of the ratio of the average particle size of the wet-process silica particles to that of the colloidal silica is from 2.0 to 30.0.
 4. The polishing composition according to claim 1, wherein a total concentration of the colloidal silica and the wet-process silica particles is 1 to 50 mass %, and the colloidal silica accounts for 5 to 95 mass % and the wet-process silica particles account for 5 to 95 mass %, of the whole of the colloidal silica and the wet-process silica particles.
 5. The polishing composition according to claim 1, wherein the water-soluble polymer compound is a copolymer further having a constituent unit derived from a hydrophobic monomer.
 6. The polishing composition according to claim 1, wherein the water-soluble polymer compound has an average molecular weight of 500 to
 20000. 7. The polishing composition according to claim 1, wherein a content of the water-soluble polymer compound is 0.0001 to 1.0 mass %.
 8. The polishing composition according to claim 1, wherein the composition is an aqueous composition further comprising an acid and an oxidizing agent, and has a pH value is 0.1 to 4.0.
 9. The polishing composition according to claim 1, wherein the composition is used for polishing an aluminum magnetic disk substrate that is plated with electroless nickel-phosphorus.
 10. A method for polishing a magnetic disk substrate, wherein the magnetic disk substrate is polished using the polishing composition according to claim
 1. 